1. Field of the Invention
The present invention relates to a flushing process in an inkjet printing apparatus having a fixed print head.
2. Description of the Background Art
There has been a widespread proliferation of inkjet printing apparatuses (or so-called inkjet printers) of the type in which printing is done on predetermined paper by the ejection or the like of fine (misty) ink particles (referred to also as ink droplets and the like) from a plurality of nozzle tips provided in a print head and having a very small diameter. In terms of print head configurations, the inkjet printing apparatuses are classified broadly into two types: a movable-head printing apparatus which performs printing by ejecting ink droplets in succession while moving a print head responsible for printing; and a fixed-head (line-head) printing apparatus which performs printing on a line-by-line basis while feeding a printing paper sheet immediately under a fixed print head having a size large enough to cover the width of the printing paper sheet.
The inkjet printing apparatuses of either type suffer from a print defect resulting from improper ejection and the like if the drying of ink due to the evaporation of a solvent near nozzles, the entry of bubbles into the nozzles or the deposition of dust onto the nozzles takes place during printing.
Some of the inkjet printers are intended to eliminate the ejection defect by executing a wipe process for wiping tip portions (nozzle portions) of the print head and a suction process for sucking up an unwanted substance from the tip portions. However, the execution of these processes requires the inkjet printers to have mechanisms therefor. Additionally, the need to execute a different operation than a printing process results in the decrease in throughput. In the use of a fixed line head of large size, a movement operation just for executing the process for such maintenance requires considerable time, which is not efficient in cost.
To avoid such problems, an inkjet printing apparatus configured to carry out flushing (also known as idle ejection, preliminary ejection and the like) during printing is known in the art, the flushing being the process of forcibly executing an ink ejection operation independent of what is to be printed under predetermined conditions. Such inkjet printing apparatuses are disclosed, for example, in Japanese Patent Application Laid-Open No. 2003-39703, Japanese Patent Application Laid-Open No. 2002-225301, Japanese Patent Application Laid-Open No. 2003-127429, Japanese Patent Application Laid-Open No. 55-139269 (1980), and Japanese Patent Application Laid-Open No. 9-216388 (1997).
The ink ejection operation executed by the printing apparatus during the flushing process is essentially identical with the ejection operation during the printing process. It is hence unnecessary to add a special mechanical component to the inkjet printing apparatus in order to execute the flushing process. A more important consideration is when to perform the flushing. Specifically, there arises a need to determine the position and pattern of ink ejection on a printing paper sheet so as not to interfere with what is to be originally printed. Additionally, more ink than necessary need not be ejected in ordinary cases because it is only necessary to prevent the drying of ink and the like.
Japanese Patent Application Laid-Open No. 2003-39703, Japanese Patent Application Laid-Open No. 2002-225301 and Japanese Patent Application Laid-Open No. 2003-127429 disclose the ejection of ink onto various regions (a perforated tear-off region, a binding margin region, a region between images, a region for bleeding and the like) of a printing paper sheet. Japanese Patent Application Laid-Open No. 2003-127429 also discloses a technique such that a pattern formed by the flushing is used as a mark for bleeding of a printed sheet. These disclosed techniques present a problem in that it is sometimes impossible to perform the flushing process because of the absence of the regions to be subjected to the ink ejection depending on what is to be printed. When such techniques are applied to the fixed-head inkjet printing apparatus, in particular, all of the nozzles arranged in line in the print head must stand ready to perform the flushing process at some point in time. This imposes a limit on when to eject the ink. As a result, there are cases where effective flushing is not carried out.
Japanese Patent Application Laid-Open No. 55-139269 (1980) discloses a technique in which the ink ejection by flushing is performed discretely on a printing sheet so that the ejected ink is inconspicuous on the printed sheet. This technique, however, may be undesirable in the case of low-resolution printing in which the dots are large in size because of conspicuity of the dots.
Japanese Patent Application Laid-Open No. 9-216388 (1997) discloses a technique employed for an inkjet printing apparatus capable of four-color printing using CMYK. In this inkjet printing apparatus, inks for CMY images are ejected for flushing from nozzles onto a position where ink for a K image is to be ejected and inks for CMY images are not in use, so as to be concealed under the ink for the K image, so that the flushing is performed in an inconspicuous manner. This technique, however, does not perform the idle ejection of the ink for the K image onto the printing sheet.